The requirements for T lymphocyte regulation of the homogenous antibody response to phosphorylcholine are being examined. The expression of the dominant T15 idiotype is subject to T15-specific helper T cell influence and anti-T15 induced T suppressor cell activity. The induction of both these cells has been found to require circulating T15 plus anti-phosphorylcholine antibody. Conventional helper cells appear to synergize with T15-specific T helper cells to induce T15 plus B cells to antibody production. These two helper populations belong to qualitatively different subpopulations.